Comparison of the Effects of Two Different Drinks on Microhardness of a Silorane-based Composite Resin.

Statement of the Problem Acidic foods and drinks can erode composite resins. Silorane-based composite is a new low shrinkage composite with higher hydrophobicity which might resist the erosive effect of beverages. Purpose The aim of this study was to determine the effects of 100% orange juice and non-alcoholic carbonated beer on microhardness of a silorane-based composite in comparison with two methacrylate-based composite resins. Materials and Method Ninety disc-shaped composite specimens were fabricated of Filtek P90, Filtek Z350 XT Enamel and Filtek Z250 (3M-ESPE) (n=30) and randomly divided into 3 subgroups of 10.Group 1 was immersed in distilled water, group 2 in 100% orange juice, and group 3 in non-alcoholic beer for 3 h/day. Primary, secondary and final Vickers microhardness tests were performed at the beginning of the study and 7 and 28 days later. Surface of 2 specimens in each group was evaluated under scanning electron microscope on day 28. Data were analyzed using repeated measures of ANOVA model (α=0.05). Results The primary and secondary microhardness of P90 was significantly lower than that of Z350, and Z250 (p< 0.001). Microhardness of Z350 was also lower than that of Z250 (p= 0.002). On day 28, microhardness of P90 was lower than Z250 and Z350 (p< 0.001); however, microhardness values of Z250 and Z350 were not significantly different (p= 0.054). Microhardness of specimens subjected to non-alcoholic beer was significantly lower than that of controls (p= 0.003). Meanwhile, the microhardness value of resins in orange juice was somewhere between the two mentioned values with no significant difference with any of them (p> 0.05). Conclusion Although 28 days of immersion in 100% orange juice and non-alcoholic beer decreased the microhardness of all specimens, P90 experienced the greatest reduction of microhardness and non-alcoholic beer had the highest effect on reducing microhardness.


Introduction
Clinical service of restorative materials is influenced by chemical abrasion due to exposure to endogenous factors such as the gastrointestinal acids and exogenous

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parameters such as the acidic and alcoholic beverages.
[1] Acid exposure affects the wear of composite resins. [2] Composite restorative materials might undergo destruction at the matrix/filler interface by acid attack. [3] Organic matrix of composite materials makes them more susceptible to chemical change compared with ceramic and metal restorative materials. [4] Siloranebased composite resins were recently introduced to make up for the drawback of methacrylate-based composites namely their polymerization shrinkage. Siloranebased composites undergo ring-opening polymerization through cationic mechanism. [3] The monomer of silorane-based composite is produced from the reaction of oxirane and siloxane molecules and the name is derived from the name of those two molecules. The two main advantages of these composites are low polymerization shrinkage and higher hydrophobicity. [5][6][7][8][9] These composite resins also benefit from less than 1.5% polymerization shrinkage, [8][9] low water sorption, [6,10] optimal biocompatibility, [11] adequate color stability, [12] better marginal fit, and less microleakage. [13] Due to the extensive use of resin-based restorative materials and their exposure to oral environment, their clinical survival and longevity is of utmost importance. The effect of acidic and alcoholic beverages such as Coca Cola, various alcoholic beverages and juices [3,[14][15][16] on methacrylate-based composites has been the subject of numerous investigations. The impact of acids in the composition of these beverages on methacrylate-based composite resins has also been very well investigated. [17][18] However, studies evaluating the effect of these beverages on silorane-based composite resins are scarce. [18] On the other hand, due to the health benefits of pure juices and higher popularity of non-alcoholic beverages among the Iranian population, this study sought to assess and compare the effect of orange juice, non-alcoholic beer, and distilled water on one siloranebased and two methacrylate-based composite resins by using the microhardness test.

Preparation of specimens
Disc-shaped composite specimens with 2mm thickness and 10mm diameter were fabricated by using a stainless steel mold. The mold was filled with composite resin between two transparent matrix bands according

Immersion in understudy beverages
Each group was randomly divided into 3 subgroups (n=10). Group 1 (control) was immersed in distilled water, group 2 in 100% natural noncarbonated orange juice (Sunich; Ali Fard Co., Iran) and group 3 in nonalcoholic carbonated beer (Behnoush Co.; Iran) within opaque screw-top glass vials containing 10ml of the respective solution 3h/day. At the end of 3 hours, the specimens were rinsed under running water and cleaned with a very soft tooth brush. They were then stored in distilled water at room temperature. Distilled water was refreshed daily in all groups. By use of a digital pH-meter before immersion, the pH of solutions was measured to be around 3.7 for orange juice and 3.3 for non-alcoholic beer.

Microhardness test
Microhardness of samples was measured in 24h (baseline), 7 days and 28 days by using a digital microhardness tester (Vickers; KB HardWin XL, KB Pruftechnik GmbH, Germany). A 100g load was applied for 20s by the indenter of Vickers machine at room temperature.
Three indentations were made on each sample with more than 1mm distance from each other at different areas of the specimen surface and the mean microhardness was calculated using the 3 obtained values.
The Vickers microhardness was calculated by measuring the diagonal lengths of each indentation through the following equation [19] HV=1.854F.d 2 where F is the applied load and d is the average of diagonal lengths of the indentation.

SEM analysis
Two specimens in each group were prepared for obser-

Microhardness test
The mean microhardness values are shown in Table 1.
Repeated measures ANOVA demonstrated that the interaction between the type of composite and microhardness changes was statistically significant ( p < 0.001). Thus, two-way ANOVA was applied which revealed that at baseline, the effect of interaction of independent variables on microhardness was not significant (p= 0.326). The effect of type of beverage on microhardness was not significant either (p= 0.998).
But, the effect of type of composite on microhardness was statistically significant (p< 0.001). Microhardness of P90 was lower than that of Z350 and the microhardness values of both were lower than that of Z250. On day 7, the effect of interaction of independent variables on microhardness was not significant (p= 0.886).
The effect of type of beverage on microhardness was not significant either (p= 0.328). However, the effect of type of composite on microhardness was statistically significant (p< 0.001). Microhardness of P90 was lower than that of Z250 and Z350 (p< 0.001). Microhardness value of Z350 was also lower than that of

SEM results
The images taken before and after immersion are pre-

Discussion
Baseline microhardness of P90 was lower than that of methacrylate-based composite resins. Filtek P90 is  properties similar to those of hybrid and midi-filled composite resins. [28][29] However, its high surface/volume ratio due to the presence of silica particles may increase its water sorption and cause destruction of polymer matrix and filler interface [30][31] compromising some of its mechanical properties. [32] Considering all the above, the understudy beverages probably affected the matrix/filler interface in this composite and caused its microhardness reduction.
However, the electron microscopic images showed no surface roughness in Z350; which was probably due to the tiny filler particles in this composite and that the microhardness reduction probably occurred due to chemical softening. The electron microscopic images showed erosion of Z250 composite surface; nonetheless, its microhardness had the smallest change compared with other composites. This finding indicated that the superficial layer has undergone corrosion but less softening has occurred in the subsurface layer compared with other composites.
Excessive hydrophobicity is another characteristic of P90 and is attributed to the presence of siloxane molecule in its chemical formulation that causes its insolubility. [11] However in our study, the microhardness of P90 decreased after immersion in the solutions. Chemical softening occurs when the solubility parameter of the resin matrix of composites is similar to that of active materials. [33] No precise information is available on the solubility parameter of silorane, but the microhardness reduction in P90 was probably due to having a solubility parameter close to that of acids present in the understudy solutions. On the other hand, it has been confirmed that weak acids such as citric acid could cause degradation of inorganic fillers [34] which might play an important role in microhardness reduction. [35] The electron microscopic images demonstrated that P90 composite surface was pitted after immersion; which probably confirms the abovementioned statement. Moreover, low pH may also be responsible for filler surface erosion and accelerated debonding of filler particles. [36] Conclusion Under the limitations of this study, although 28 days of immersion in 100% orange juice and non-alcoholic beer decreased the microhardness of all specimens, P90 experienced the greatest reduction in microhardness and non-alcoholic beer had the highest effect on reducing microhardness.